This invention relates to a two-way automated communications system (TWACS) used by electrical utilities for sending messages from the utility to an end device, and for receiving and processing responses from the end device containing information relating to matters such as current energy consumption, power outages, etc. More specifically, the invention sets forth a method for improvement in the detection for outbound messages sent by the utility.
TWACS messages, which can be categorized as either outbound messages (OM) or inbound messages (IM), are sent and received over a selected phase (φ) of the electrical distribution system. When an outbound message is sent, the transponder detects the message and validates it using channel coding. As discussed hereinafter, synchronization to the outbound message signal involves matching a 9-bit preamble of the message against an expected bit pattern. It will be understood that synchronization does not necessarily mean the message detected by the transponder will pass error detection as part of the channel coding. Furthermore, because the entire message has not yet been received at the time of synchronization, the transponder must continue to detect bits until the end of the message is received. If the transponder erroneously detects what appears to be a preamble, it will attempt to detect the bits following the preamble, even if no signal is present. This can result in the missed detection of an outbound signal that occurs while the transponder is in the bit detection state.
Sometimes an erroneous preamble detection can be triggered in a transponder by random noise on the power line, but a far more likely cause is TWACS inbound signals. There is sufficient similarity between TWACS inbound signals and TWACS outbound signals that an inbound signal can contain a bit pattern that matches the preamble of an outbound message. Depending on system configuration, it is possible for such patterns to occur immediately before an outbound message is sent over the same phase as a particular transponder.
Similarly, a transponder can miss a TWACS outbound message when it is preceded by another outbound message. Consecutive outbound messages can be sent on the same phase or different phases. If a transponder incorrectly synchronizes to the first outbound message (due to other noise sources) the transponder is more likely to miss the second outbound message as well.
This problem is compounded because once a transponder synchronizes to a bit pattern that matches the outbound preamble, the transponder stops looking for any other preamble patterns. It will be understood that the bit pattern detected could be incorrect as well as correct. As the transponder continues processing what it now believes is an outbound transmission based upon synchronization to an incorrect bit pattern, a blind spot (time period) is created in the transponder. If a true outbound message is transmitted on the phase of the transponder during this period, it will miss the message because it is not looking for it during that time. The blind spot may last anywhere from a few bytes of TWACS outbound message time (for the shortest valid message), up to the maximum message size of 31 bytes. This corresponds to a range in time of between 2.13 seconds and 16.50 seconds.
While inbound signals are a common source of false outbound preambles that cause the outbound detector to miss true outbound signals, the problem can also be triggered by power line noise. When no signals are present and the transponder is hunting for a preamble, there is a certain probability that any random noise sequence will match the outbound preamble pattern. Again, this may result in the transponder processing what it believes is the outbound message without further watching for a true outbound message.